A time-temperature indicating device often accompanies a shipment of perishable items so as to provide a record of the temperatures those items have been exposed to during their journey. For example, such devices are typically included with shipments of fresh and frozen foods, photographic materials, and/or pharmaceutical products. These and other products are often sensitive to small increases in storage temperature, even if they are held at the elevated temperature for only a short period of time. Absent a time-temperature indicating device, the receiver of such products may not be able to determine whether those products have been exposed to harmful elevated temperatures, or the duration of that exposure.
Relatively expensive temperature recording devices, that are capable of recording temperature on a time basis, are well known. For example, chart recorders and/or electronically controlled temperature recording devices are available that can record variations in the ambient temperature on a time basis. Unfortunately, these devices are often too costly to be considered for individual containers, or in particular, for disposable applications.
Disposable devices capable of indicating when a specific temperature has been reached are also well known in the art. For example, U.S. Pat. No. 3,954,011, issued May 4, 1976, to Manske, teaches an indicating device suitable for visibly indicating time, temperature, and time-temperature relationships. The device consists of a porous fluid carrying pad, a wick material, and an indicating means. The device is adapted to regulate the migration of the fluid from the fluid source to the wick material as a function of temperature. Calibrated indicating means are provided to show the progress of the fluid along the wick material as a function of time. In this way, the device is adapted to indicate and measure the passage of time, the exposure to a given minimum temperature, or a time-temperature relationship. Unfortunately, such devices are frequently susceptible to tampering, thus rendering them unreliable. Additionally, devices of this type many times do not give an immediate indication of the specific temperature level that has been attained or the time interval over which that temperature exposure occurred.
As a result of these deficiencies in the prior art, inexpensive time-temperature indicators have been devised to minimize the foregoing problems. For example, U.S. Pat. No. 4,488,822 discloses a time-temperature indicator comprising first and second compartments that are in fluid communication with one another. A first liquid, having a predetermined freezing point, is disposed in a portion of the first compartment. The second compartment is adapted to contain a pressurized fluid. In operation, the first liquid is forced out of the first compartment, at a controlled rate by the pressurized fluid in the second compartment, when the ambient temperature is above the freezing point of the first liquid. Flow control means, such as a porous barrier, are disposed between the first and second compartments so as to regulate the rate at which the pressurized fluid forces the first fluid through the first compartment. Manual means are provided for pressurizing the second fluid.
Such devices are capable of indicating the interval of time that a product has been exposed to temperatures above a predetermined temperature. Unfortunately, these devices invariably use fluids that must be frozen, i.e., be maintained at or below 0.degree. C. prior to use. Thus these devices often incorporate water-based fluids that make them ineffective for elevated temperature environments, i.e. at or above room temperature.